Ultrasonic transducers can be used to measure a distance to the surface of a liquid. In some situations, a transducer is positioned at the top of a liquid fuel tank. An ultrasonic signal is generated by the transducer and the time it takes for the signal to travel from the top of the tank to the surface of the fuel, reflect off the surface of the fuel, and return to the transducer is measured. If certain information about the tank is known, such as its volume or dimensions, the time measurement can be used in a calculation to determine how much fuel is in the tank.
A variation of such a fuel or liquid level measurement device or system involves placing a transducer at the bottom of a tank, directed upward toward the liquid's surface. A relatively high-voltage electric signal is sent to a transducer (e.g. a piezo-electric transducer) causing the transducer to change shape and resonate at its natural mechanical frequency (or at a multiple thereof). The mechanical resonance results in a short duration pulse of ultrasonic energy being directed towards the surface of the liquid. The ultrasonic sound wave travels through the liquid and reflects off a vapor/liquid interface due to differing speeds of sound between the vapor and the liquid. A portion of the reflected sound energy returns towards the transducer in the form of an echo. The ultrasonic energy imparted by the returning echo causes the transducer to resonate. In turn, this resonance is converted to a relatively low-voltage signal which is detected by measurement electronics. The time between the transmitted ultrasonic pulse and the received echo is directly proportional to the distance the sound wave traveled through the liquid as expressed in the equation below:Distance=Speed×(Time of Flight)/2Where, Speed is the speed of sound within the fluid and is a function of the temperature and the density of the liquid. The distance can be readily calculated if the speed of sound is known.